Method for purifying polycrystalline silicon

ABSTRACT

Polysilicon fragments are purified to remove metal contaminates by contacting the fragments with a purifying liquid at a flow rate &gt;100 mm/sec. Effective removal without abrasion is accomplished.

The invention relates to a method for purifying polycrystalline siliconwith an improved flow of the purifying solutions in the process.

High-purity semiconductor material is required for the production ofsolar cells or electronic components, such as memory elements ormicroprocessors for example. The dopants introduced in a targeted mannerare the only impurities that a material of this type should have in themost expedient case. Therefore, endeavors are made to keep theconcentrations of harmful impurities as low as possible. It is oftenobserved that semiconductor material that has already been produced withhigh purity is contaminated again in the course of further processing toform the target products. Therefore, complicated purifying stepsrepeatedly become necessary in order to restore the original purity.

In particular, contamination by metal atoms should be regarded ascritical since the latter can alter the electrical properties of thesemiconductor material in a harmful manner. If the semiconductormaterial that is to be comminuted is comminuted, in the manner that haspredominantly been customary heretofore, by means of mechanical tools,such as crushers made of steel, for example, then the fragments have tobe subjected to surface purifying prior to melting.

In order to remove the impurities, by way of example, the surface of themechanically processed polycrystalline silicon is etched using a mixtureof nitric acid and hydrofluoric acid. In the course of the process, themetal particles are attacked to a great extent by the acid mixtureduring the preliminary purifying. Metal carbide residues remain, and aredissolved to the greatest possible extent during the HF/HNO₃ mainpurifying.

In this case, the polysilicon fragments are usually dipped successivelyinto different purifying solutions during purifying in baskets orbasins.

EP 0905796 describes a purifying process comprising preliminarypurifying by means of a mixture comprising HF/HCl/H₂O₂, main purifyingby means of HF/HNO₃ and subsequent hydrophilization of the siliconsurface by means of HCl/H₂O₂. During the purifying process, rinsingtakes place in throughflow or dump tanks. In this case, the siliconfragments are purified in a purifying machine on the basis of an up anddown movement. In addition, the basin loaded with polysilicon fragmentscan also move completely out of the liquid during the lifting/loweringmovement, in order that the purifying solution can completely drain awayfrom the silicon fragments. A disadvantage that emerges is that spotshaving a gray appearance are found on the silicon fragments in thecourse of the process. Investigations have shown that the gray spotsalways occur at the contact points between individual poly fragments orbetween poly fragments and the process basin wall. The cause isexcessively little flow in the dead water zones between the individualpoly fragments. A further disadvantage in the method described is theundesirable residual acid concentration at the fragments. Despitegreatly increasing the flow rate up to complete liquid exchange in thetank in less than one second, it has always been possible to detect tinyacid traces in the pptw range by means of ion chromatography and ICmeasurements on the purified poly fragments.

It is also known from the prior art to etch polysilicon rods (FZ rods)and thin rods in drum apparatuses. Although experiments on apparatusesof this type with poly fragments show that no superimposed spots ariseat rotational speeds of greater than revolution per minute, thesharp-edged polysilicon fragments produce visible abrasion of the drummaterial even at a low rotational speed of the drum. This abrasion isunacceptable for subsequent application of the polysilicon fragments assemiconductor material.

US 2006/0042539 describes an apparatus in which the tray in thecontainer is caused to effect a regulated translational movement in alateral direction during the treatment duration. However, thetranslational movement leads to the same results as the lifting/loweringmovement described in EP 0905 796. The problem of the dead water zone atthe contact points cannot be solved with an apparatus of this typeeither.

DE 69905538 describes a centrifuge apparatus for purifying small parts.In this case, a rearrangement of the parts is achieved by means of acontinual change between centrifugal force and gravitation. Thecentrifugal force takes effect at high rotational speed, and thegravitation at low rotational speed. The parts are continuallyrearranged as a result of this change. What is disadvantageous here,too, is that abrasion arises as a result of the relative movement withthe vessel, said abrasion being unacceptable forsemiconductor-conforming polysilicon fragments.

None of the solutions known from the prior art leads to useable results.If the etching devices are linked with rearrangement of the polysiliconfragments, unacceptable abrasion of the containers must always bereckoned with. Applications such as are known from the purifying andetching of semiconductor wafers cannot be employed owing to thedifferent geometries of polysilicon fragments and wafers.

Therefore, the object was to provide an improved method for purifyingpolysilicon fragments in which no dead water zones occur and theformation of spots on the polysilicon fragments is thus prevented. Atthe same time, the rinsing capacity is intended to be high enough thatacid residues are no longer detectable on the polysilicon fragments.

It has surprisingly been found that in the case of etching polysiliconwith flow velocities onto the polysilicon surface of greater than 100 mmper second, impinging on the surface of the polysilicon fragments frommore than two different directions, it is possible to increase theremoval with the same acid concentration and, at the same time it hasbecome possible to eliminate the arising of dead water zones and thusthe cause of the spots.

The invention relates to a method for purifying polysilicon fragments,characterized in that the flow of the purifying liquid in at least oneof the process steps has a flow velocity of greater than 100 mm/sec,which impinges on the surface of the polysilicon fragments from morethan two different directions.

The method according to the invention has made it possible to improvethe substance exchange during the residence times of the polysiliconfragments in the purifying baths in such a way that no dead water zonesarise in the bulk material and so spots no longer arise on thepolysilicon fragments. It has become possible to significantly increasethe etching removal with the same acid concentration.

Various arrangements are possible for producing a flow of greater than100 mm/sec during the purifying process, which impinges on the surfaceof the polysilicon fragments from more than two different directions.

In one embodiment of the method according to the invention, theimprovement in the flow conditions between the contact points of theindividual fragments is obtained by means of non-directional, diffuseinjection of the purifying liquid into the etching tank, by means ofalternately active nozzles (FIG. 1). In this case, a plurality ofnozzles for introducing the purifying liquid (2) are situated in theetching tank (1), said nozzles being fitted to the base and to the sidewalls. The bulk material is suspended on the sides and on the base in abasin (3) having openings (4) in the etching tank. Numbers of nozzles ofbetween 1 and 1000 are preferred in this case. 10 to 100 nozzles areparticularly preferred. The etching mixture emerges from said nozzles ata velocity of greater than 100 mm/sec. Flow velocities of 100 to 200mm/sec are preferred, particularly preferably 150 mm/sec.

The nozzles can be opened in a temporally staggered manner in analternating cycle with a temporal delay of 0.1 to 60 sec for a time of0.1 to 60 sec. Temporal delays of 1 to 4 sec and an adjustable openingtime of 0.2 to 1 sec are preferred. The nozzles have an opening of 0.01to 5 mm. An opening of 0.5 to 2 mm is preferably used. 50 nozzles havingan exit diameter of 1 mm are particularly preferred. The alternateincident flow on the individual fragments prevents dead water zones fromarising at the contact points between the poly fragments. A uniform flowvelocity prevails at all points in the bulk material.

In a further embodiment of the method according to the invention, theimprovement in the flow conditions between the contact points of theindividual fragments is produced by one or more moved nozzle rings inthe etching tank (FIG. 2). In this case, moved, rotating nozzle rings(5) are arranged around the process basin (3). By varying the exitvelocity from the nozzles, an incident flow of greater than 100 mm/secfrom different directions on the fragments is obtained in the etchingtank in this case. The nozzle rings preferably contain between 5 and 500nozzles having an opening of 0.01 to 5 mm.

As in the embodiment already described above, the nozzles can alsoadditionally be actuated with a temporal delay and an adjustable openingtime. The times as described in the first embodiment are likewisepreferred here. 10 to 100 nozzles having an opening of 0.01 to 5 mm arepreferred. An opening of 0.5 to 2 mm is preferably used. 50 nozzleshaving an exit diameter of 1 mm are particularly preferred.

In a further embodiment of the method according to the invention, theimprovement in the flow conditions between the contact points of theindividual fragments is produced by a so-called “principle of therotating acid” (FIG. 3).

In this case, a plurality of nozzles (6) having an opening of 0.01 to 5mm are arranged at the base of the etching tank (1) in such a way thatthe acid mixture is cause to effect a rotational movement. The etchingmixture emerges from the nozzles at a velocity of greater than 100mm/sec. Preference is given to nozzles having an opening of 0.5 mm to 4mm, particularly preferably of 1 mm, and an exit velocity of 100 mm/sec.The process basin (3) can rest in the rotating acid (7) or be moved bymeans of an additional lifting/lowering movement. Preference is given toan additional lifting/lowering movement in the case of which the processbasin completely enters and exits from the liquid during eachlifting/lowering movement.

In a further embodiment of the method according to the invention, theimprovement in the flow conditions between the contact points of theindividual fragments is produced by the application of a turntable whichrotates in a horizontal plane and on which the process basin is situated(FIG. 4).

In this case, the rotational movement of the process basin (3) on theturntable (8) is preferably between 1 and 500 revolutions per minute. Asufficient incident flow on the polysilicon fragments from differentdirections is thus produced in the etching tank (1). A rotational speedof 20 to 100 revolutions per minute is particularly preferred,especially preferably 50 revolutions per minute. The setting of thesuitable rotational speed of the horizontal rotatary movement producesan incident flow from different directions onto the surface of theindividual silicon fragments at a velocity of greater than 100 mm/sec.

In a further embodiment of the method according to the invention, theimprovement in the flow conditions between the contact points of theindividual fragments can also be achieved by additional, non-directionalinjection of air through the base of the basin (FIG. 5). This measureresults in an increase in the flow velocity at the critical contactpoints of greater than 100 mm/sec. Preferably, 5 to 100 nozzles (9) arefitted to the base of the etching tank (1), from which nozzles the airis injected into the etching tank from below in the direction of theprocess basin (3) with the polysilicon fragments (9). The size of theopening of the nozzle outlets is preferably 0.01 to 5 mm. The pressureof the injected air is preferably between 0.1 and 200 bar. 20 to 100nozzles having an opening of 0.1 to 1 mm nozzle opening are particularlypreferred.

As in the embodiments described above, the nozzles can additionally alsobe actuated with a temporal delay and an adjustable opening time.Preference is given to the temporal delays and the opening timesanalogously to the embodiments already described. As a result of theadditional turbulence produced by the injected air into the purifyingsolution, the acid can flow through unimpeded at all the contact pointsbetween the poly fragments.

In a further embodiment of the method according to the invention, theimprovement in the flow conditions between the contact points of theindividual fragments is produced by a process basin that moves on avertical axis (FIG. 6).

For this purpose, the process basin (3) used is equipped with lateralholes (10) and is led through the etching bath on a vertical circularpath. In this case, a circular movement with a frequency of 1 to 200revolutions per minute is preferred, and a rotational speed of 5 to 50revolutions per minute is particularly preferred, especially preferably10 revolutions per minute.

The circular movement can be carried out within the purifying liquid orelse partially outside the purifying liquid. If a circular movement isperformed in the course of which dipping into and out of the liquidtakes place, the process basin can in this case be moved wholly orpartly out of the liquid.

Preference is given to a circular movement with dipping into and out ofthe liquid, the process basin being continually filled and emptied.

In a further embodiment of the method according to the invention, theimprovement in the flow conditions between the contact points of theindividual fragments is produced by applying ultrasound. Surprisingly,it was possible to show that by applying ultrasound having an operatingfrequency range of 10 kHz to 5 GHz the attack on silicon in the HF/HN03etchant is surprisingly considerably increased. An operating frequencyof 500 kHz to 2 GHz is preferred. An operating frequency of 1 GHz isparticularly preferred. In the bulk material the ultrasound has apositive effect on the etching process and on the dissolution of themetal particles. The metal surface values can be significantly reducedand it was thus possible to obtain the same effect as at flow velocitiesof greater than 100 mm/sec.

Surprisingly, using the ultrasound technique, acid residues can bebetter rinsed out of cracks in poly fragments in both the acid and therinsing baths.

All the arrangements according to the invention described made itpossible for the purifying liquid to flow through the polysilicon bulkmaterial from more than 2 different directions at a velocity of greaterthan 100 mm/sec. In this case, there is uniform flow through the contactpoints in the bulk material and, on the purified polysilicon, spots donot occur on the fragments nor can appreciable acid residues bedetected.

The invention will be explained in greater detail on the basis of thefollowing examples.

Carrying Out the CE Measurements/Ion Chromatography Measurements:

5 kg of poly fragments were filled in a closed plastic container with100 ml of ultrapure water. The closed container is left for one week.Afterward the fluoride, nitrate, nitrite and chloride content wasmeasured by means of an ion chromatography measurement or capillaryelectrophoresis.

Carrying Out the Metal Analyses on the Purified Poly Fragments:

In a Teflon funnel, polysilicon having a weight of 100 g was sprayedwith 40 ml of HF/HN03 1:4. The etching acid was collected in a Teflonbowl. Afterward, the acid was evaporated and the residue taken up in 5ml of water. The metal content of the aqueous solution is measured onthe ICP-AES (inductively coupled ion plasma atomic emission spectroscopefrom Spectro). The metal content of the polysilicon surface iscalculated from the measured values. The data were given in pptw.

COMPARATIVE EXAMPLE 1 Influence of the Flow on the Etching Removal andSpots in a Lifting/Lowering Apparatus

The influence of the flow in the etching tank on the etching removal andthe spots was investigated on the basis of the lifting/lowering methodfor purifying poly fragments as described in EP 0905 796. In thismethod, the flow onto the polysilicon fragments is produced by an up anddown movement of the process basin filled with poly fragments. The acidcirculates in a vertical direction from the bottom upward.

The relationship between the flow onto the poly surface that resultsfrom the up and down movement and circulation, and the etching removalcan be gathered from Table 1 below.

Purifying solution used: 5% by weight HF, 55% by weight HNO₃ and 8% byweight H₂SiF₆; temperature in the etching bath 20° C.

TABLE 1 Total of the flow onto the poly fragments Silicon removal  0mm/sec 0.6 μm/min. 10 mm/sec 2.5 μm/min. 30 mm/sec 4.3 μm/min. 50 mm/sec5.5 μm/min. 70 mm/sec 5.7 μm/min. 90 mm/sec 5.8 μm/min. 100 mm/sec  5.8μm/min.

Purifying solution used: 5% by weight HF, 55% by weight HNO₃ and 8% byweight H₂SiF₆; temperature in the etching bath 20° C.

The table shows that with a lifting/lowering apparatus, at a flow ofgreater than 50 mm/sec onto the polysilicon surface, the etching removalno longer appreciably increases. With a lifting/lowering apparatus, flowvelocities up to a maximum 100 mm/sec are possible on an industrialscale for production installations with a tenable financial outlay. Atflow velocities up to 100 mm/sec, however, gray spots are obtained as aresult of the excessively small substance exchange in the dead waterzones.

The purified poly fragments contained the following analysis values fromion chromatography or CE measurements: fluoride 2 pptw, nitrate 5 pptw,nitrite 0.1 pptw and chloride 3 pptw.

EXAMPLE 1 Flow Through Alternate Nozzles

A polysilicon rod was comminuted and classified by means of a devicecomprising a comminuting tool and a screening device. 5 kg of polyfragments were treated in a process basin using the following 3-stagepurifying process analogously to EP 0 905 796. For preliminarypurifying, the polysilicon fragments were purified for 20 minutes in amixture comprising HF/HCl/H₂O₂ at a temperature of 25° C. During thesubsequent main purifying, the polysilicon fragments were etched for 5minutes at 8° C. in a mixture of HF/HNO₃. This was followed by rinsingfor 5 minutes in ultrapure water with 18 megohms at a temperature of 22°C. Finally, hydrophilization was effected for 5 minutes in a mixturecomprising HCl/H₂O₂ at a temperature of 22° C. and drying was effectedfor 60 minutes using ultrapure air of class 100 at 80° C.

During the preliminary purifying, in the rinsing baths and during thehydrophilization, the basket containing a weighed-in quantity of 5 kgcarried out an up and down movement with the poly fragments with astroke frequency of 5 strokes per minute.

The main purifying took place in an etching bath with 500 l of acid, inwhich 50 nozzles were situated. The HF/HNO₃ etching mixture emerged fromthe nozzles at a velocity of 150 mm/sec. The nozzles were opened intemporally staggered fashion in an alternating cycle with a temporaldelay of 2 sec for a time of 0.5 sec. The nozzles had an opening of 1mm. The alternate incident flow on the individual fragments preventeddead water zones from arising at the contact points between the polyfragments. A uniform flow velocity prevails at all points in the bulkmaterial. After the hydrophilization and drying, poly fragments withoutspots on their lustrous surfaces were obtained.

EXAMPLE 2 Etching in Rotating Acid

A procedure analogous to example 1 was employed.

In a departure therefrom, during the main purifying, the process basinin a 500 liter HF/HNO₃ etching mixture in the acid tank was moved with alifting/lowering movement at a frequency of 5 strokes per minute. Fournozzles having an exit opening of 1 mm were fitted at the base of thetank. The acid emerged from said nozzles at a velocity of 150 mm/sec.During the lifting/lowering movement, the basin continually completelyemerged from the acid and entered it again. The acid had a temperatureof 8° C. As a result of the rotating acid, a uniform throughflow wasachieved at all points in the bulk material. This made it possible toprevent dead water zones at the contact points between the polyfragments.

After the hydrophilization and drying, poly fragments without spots ontheir lustrous surfaces were obtained.

EXAMPLE 3 Horizontal Turntable

A procedure analogous to example 1 was employed.

In a departure therefrom, during the main purifying, the process basinin a 500 liter HF/HNO₃ etching mixture in the acid tank was rotated on aturntable in a horizontal direction. The rotary movement was 50revolutions per minute. An incident flow from different directions ontothe surface of the individual fragments of 150 mm/sec was produced bythe horizontal rotary movement at the rotational speed specified. Thismade it possible to prevent dead water zones at the contact pointsbetween the poly fragments. After the hydrophilization and drying, polyfragments without spots on their lustrous surfaces were obtained.

EXAMPLE 4 Diffuse Injection of Air

A procedure analogous to example 1 was employed.

In a departure therefrom, during the main purifying, the process basinin a 500 liter HF/HNO₃ etching mixture in the acid tank was moved with alifting/lowering movement at a frequency of 5 strokes per minute. 50nozzles having an opening of 0.1 mm were additionally fitted at the baseof the tank. Through these nozzles, air was additionally injectedthrough the base of the basin. This made it possible to achieve anincrease in the flow velocity at the contact points. The flow velocitywas 150 mm/sec. The nozzles were opened with a temporal delay of 2 secfor 0.5 sec. As a result of the additional turbulence produced by theinjected air, the acid can flow through unimpeded at all contact pointsbetween the poly fragments.

After the hyrophilization and drying, poly fragments without spots ontheir lustrous surfaces were obtained.

EXAMPLE 5 Vertical Movement of the Process Basin

A procedure analogous to example 1 was employed.

In a departure therefrom, during the main purifying, the process basinin a 500 liter HF/HNO₃ etching mixture was moved with a verticalcircular movement through the acid tank. The frequency of the circularmovement was 10 revolutions per minute. As a result, the acid flows ontothe poly surfaces at a velocity of 150 mm/sec from all directions. Thebath temperature was 8° C. and there was continuous circulation. Thetime in the etching bath was 5 minutes. During the circular movement,the basin was completely immersed in the liquid and completely removedtherefrom in each cycle. The circular movement results in uniform flowthrough the bulk material which prevents dead water zones from arisingat the contact points of the poly fragments. After the hydrophilizationand drying, poly fragments without spots on their lustrous surfaces wereobtained.

EXAMPLE 6 Flow Using Ultrasound

A polysilicon rod was comminuted and classified by means of a devicecomprising a comminuting tool and a screening device. 5 kg of polyfragments were treated in a process basin using the following 3-stagepurifying process analogously to EP 0 905 796. For preliminarypurifying, the polysilicon fragments were purified for 20 minutes in amixture comprising HF/HCl/H₂O₂ at a temperature of 25° C. During thesubsequent main purifying, the polysilicon fragments were etched for 5minutes at 8° C. in a mixture of HF/HNO₃. This was followed by rinsingfor 5 minutes in ultrapure water with 18 megohms at a temperature of 22°C.

Finally, hydrophilization was effected for 5 minutes in a mixturecomprising HCl/H₂O₂ at a temperature of 22° C. and drying was effectedfor 60 minutes using ultrapure air of class 100 at 80° C.

During the preliminary purifying, during the HF/HNO₃ main purifying, inthe rinsing baths and during the hydrophilization, the basket filledwith 5 kg of poly fragments carried out an up and down movement with astroke frequency of 5 strokes per minute. An ultrasonic generator havingan operating frequency of 1 GHZ was additionally incorporated in allpurifying and rinsing steps.

Poly fragments with fewer particles in comparison with a process withoutan ultrasound bath and with a lower metal level on the poly surface wereobtained after the end of the process. The purified polysiliconfragments had no spots on their lustrous surfaces.

EXAMPLE 7 Rinsing Bath with Ultrasound

In an ultrasound bath, poly fragments were introduced into a plastic tubwith 18 Mohm water for several minutes. By applying ultrasound with anoperating frequency range of 3 GHZ, it was possible to remove acidresidues situated in a rough poly surface having cracks smaller than 5μafter etching using HF/HNO₃. As a comparison, an attempt was made tocompletely remove acid residues on the poly fragments in a normalthroughflow tank with 18 Mohm water. The residual acid content from bothapproaches was determined by the ion chromatography method or the CEmethod.

Cracked poly from Cracked poly from ultrasonic normal rinsing rinsingFluoride 2 pptw Fluoride <DL (1 pptw) Nitrate 5 pptw Nitrate <DL (2pptw) Nitrite 0.1 pptw   Nitrite <DL (0.01 pptw) Chloride 3 pptwChloride <DL (1 pptw)

EXAMPLE 8 Rinsing Baths with 3 GHZ Ultrasound

A polysilicon rod was comminuted and classified by means of a devicecomprising a comminuting tool and a screening device. 5 kg of polyfragments were treated in a process basin by means of the followingpurifying process.

For preliminary purifying, the polysilicon fragments were purified for20 minutes in a mixture comprising 5% by weight HF, 8% by weight HCl and3% by weight H₂O₂ at a temperature of 25° C. The removal of thepolysilicon surface was 0.02 p in this case.

Afterward, rinsing was effected for 5 minutes at 3 m³/hr in anultrasound bath with plastic lining at 3 GHZ at 22° C. During thesubsequent main purifying the polysilicon fragments were etched for 5minutes at 8° C. in a mixture of HF/HNO₃ comprising 3% by weight HF, 65%by weight HNO₃. The etching removal was approximately 12 μm.

Afterward rinsing was effected for 5 minutes at 2 m³/hr in an ultrasoundbath with a plastic lining at 3 GHZ at 22° C. The polysilicon fragmentswere subsequently hydrophilized in a further step in a mixturecomprising HCl/H₂O₂ comprising 8% by weight HCl and 2% by weight H₂O₂for 5 minutes at 22° C.

Afterward, rinsing was effected for 5 minutes at 1 m³/hr at 3 GHZ at 22°C. and at 80° C. for a further 5 minutes at 4 m³/hr in an ultrasoundbath with plastic lining. Drying was then effected for 60 minutes usingultrapure air of class 100 at 80° C.

Ion chromatography measurements or CE measurements show that thepurified poly fragments contain acid residues below the detection limit.

The following metal surface values were obtained in this case:

Element Concentration Element Concentration Fe 13 pptw  Ti 8 pptw Cr 1pptw W 1 pptw Ni 1 pptw K 5 pptw Na 12 pptw  Co 0 pptw Zn 8 pptw Mn 0pptw Al 8 pptw Ca 25 pptw  Cu 2 pptw Mg 8 pptw Mo 0 pptw V 0 pptw

COMPARATIVE EXAMPLE 2 Rinsing Baths with 3 GHZ Ultrasound

A procedure analogous to example 8 was employed, but the assistance ofultrasound was dispensed with during the rinsing steps.

Ion chromatography measurements or CE measurements show that thepurified poly fragments still contain acid residues.

The following metal surface values were obtained in this case:

Element Concentration Element Concentration Fe 13 pptw  Ti 23 pptw  Cr 1pptw W 1 pptw Ni 1 pptw K 5 pptw Na 24 pptw  Co 0 pptw Zn 8 pptw Mn 0pptw Al 28 pptw  Ca 45 pptw  Cu 2 pptw Mg 11 pptw  Mo 0 pptw V 0 pptw

1.-9. (canceled)
 10. A method for purifying polysilicon fragments in anetching tank, comprising impinging a purifying liquid in at least one ofthe process steps onto the surface of the surface of the polysiliconfragments from more than two different directions at a flow velocity ofgreater than 100 mm/sec.
 11. The method of claim 10, wherein thepurifying liquid impinges on the surface of the polysilicon fragments bynon-directional, diffuse injection of the purifying liquid by means ofnozzles.
 12. The method of claim 10, wherein the purifying liquidimpinges on the surface of the polysilicon fragments via one or moremoved nozzle rings in the etching tank.
 13. The method of claim 10,wherein the purifying liquid is caused to assume a rotational movementby virtue of the arrangement of the nozzles in the etching tank.
 14. Themethod of claim 10, wherein the polysilicon fragments rotate in ahorizontal or vertical direction in a holding device within thepurifying liquid.
 15. The method of claim 10, wherein the purifyingliquid is caused to effect a flow by additional, non-directionalinjection of air by means of nozzles.
 16. The method of claim 10,wherein the flow velocities and flow directions are produced by the useof ultrasound.
 17. The method of claim 10, wherein the silicon fragmentsare additionally moved in a lifting/lowering movement in the etchingtank.
 18. The method of claim 17, wherein the silicon fragments arewholly or partly removed from the purifying solution during thelifting/lowering movements.